1. Field of the Invention
The present invention relates to a power supply unit for supplying electric power to an electronic apparatus connected to the power supply unit, and a system comprising the power supply unit and the electronic apparatus.
More particularly, the present invention relates to a power supply unit which is connected to an electronic apparatus such as a recording apparatus, incorporates a battery or cells, and can selectively effect supply and cutoff of power to the electronic apparatus. The present invention also relates to a system comprising the power supply unit and the electronic apparatus.
2. Description of the Related Art
Recently, a variety of carrying-type electronic apparatuses such as portable personal computers, cellular phones, video cameras and portable printers have been put into the market. Those electronic apparatuses are usable without cords in order that users can carry the apparatuses and operate them any places. Therefore, those electronic apparatuses are constructed to be capable of operating with a battery or cells incorporated therein without being connected to domestic power sources through power cords.
In these days, the so-called secondary cells, which can be recharged repeatedly, have been employed as power sources for many of those electronic apparatuses. Known examples of the secondary cells are nickel cadmium cells, nickel hydrogen cells, lithium ion cells, etc.
When the secondary cells are employed as power sources of those electronic apparatuses, it is not usual that the power source comprises a single cell. Generally, a plurality of cells are combined in series or parallel and housed in a case to construct a dedicated battery pack which is incorporated in or connected to the electronic apparatus.
Of the secondary cells, lithium ion cells are advantageous in having a higher energy density per predetermined unit of volume and a higher energy density per predetermined unit of weight than other secondary cells such as nickel cadmium cells and nickel hydrogen cells. In the case of using lithium ion cells, it is known to incorporate a protection circuit in a battery pack for increasing safety of the cells against overcharge or supply of overvoltage.
FIG. 3 is a circuit block diagram of a conventional battery-driven system comprising a power supply unit which incorporates a battery (cells) therein to supply electric power, and a main unit which operates with the power supplied as a drive source from the power supply unit. In FIG. 3, reference numeral 10 on the left side denotes a battery pack constituting the power supply unit. Three lithium ion cells 11 are housed in the battery pack 10 and connected in series. Also, a discharge switch 12 and a charge switch 13 are provided in the battery pack 10. As shown in FIG. 3, the discharge switch 12 and the charge switch 13 are disposed in a current path through which the cells are charged and discharged, and are connected in series. Additionally, the battery pack 10 is provided with output terminals V and E for outputting the power. The battery pack 10 further includes a protection circuit for maintaining safety of the cells. The protection circuit functions such that if there occurs any situation affecting safety of the cells, such as overdischarge, overcurrent, overvoltage or overcharge, the discharge switch 12 and the charge switch 13 are turned off by a control circuit 14 to cut off the power outputted across the output terminals V-E.
Reference numeral 20 on the right side of FIG. 3 denotes the main unit working with the battery pack 10 used as a power supply for the work. The power supplied from the battery pack 10 is applied to a DC--DC converter 22 through an FET (Field Effect Transistor) 21 for conversion into a source voltage Vcc adapted for logics. The main unit 20 comprises a microprocessor (MPU) 23, a ROM 24 for storing a control program used for control executed by the MPU 23, and a RAM 25 serving as a work area for execution of the control program. The operation of the entire main unit 20 is controlled by the MPU 23.
The main unit 20 includes a power switch 26 for selectively turning on/off the source power. The power switch 26 comprises, for example, a tactile switch. A gate of the FET 21 is controlled to close or open upon the power switch 26 being turned on/off. An on/off state of the source power of the main unit 20 is controlled corresponding to control of the gate of the FET 21. An output signal from the power switch 26 is read into the MPU 23 through an input port 27, while the on/off state of the FET 21 is controlled in response to an output signal from an output port 28 through a transistor 30. When the source power of the main unit 20 is turned off, the output from the battery is perfectly cut off and the battery is prevented from discharging.
However, the conventional battery-driven system thus constructed has experienced the following problems.
The FET 21 serving as a switch for turning on/off the source power of the main unit 20 is inserted in a power line in series. Looking from the battery side, therefore, FETs are inserted double in the power line because the discharge switch 12 and the charge switch 13 incorporated in the battery pack 10 each usually comprise an FET and the FET 21 is provided in the main unit 20. With such a construction, resistance developed at the time of turning on the FETs is increased and ineffective Joule heat is generated. Also, an increase of the resistance produces a voltage drop, thus resulting in a reduction of battery efficiency and shortening of battery life.
The above problem becomes more important when a recording apparatus requiring a larger current is used as the main unit 20 which is connected to the battery pack 10. In thermal printers and ink jet printers which require a relatively large current, particularly, in ink jet printers wherein ink is ejected by heat generated by heaters, shortening of battery life leads to a remarkable reduction of operating time per charge, thus resulting in a critical problem.
Another problem in the conventional battery-driven system having the above-described construction is that it has been difficult to reduce the apparatus size and cut down the cost because the FET 21 or the like must be provided as a switch on the side of the main unit 20.